Bridge plugs are tools that are lowered into a cased oil or gas well. When set in position inside of the casing, a bridge plug provides a seal to isolate pressure between two zones or intervals in the well. Bridge plugs are useful during workover operations when, for whatever reason, a seal is required downhole to isolate two zones. An example of such a situation occurs during workover operations on an upper zone, where the lower zone generates high pressure. The bridge plug acts as a stop to prevent the leakage of fluid from the lower zone into the upper interval of the well.
Retrievable bridge plugs are used during workover operations when a temporary separation of zones is required. A permanent type of bridge plug could be used; however, this type of bridge plug is removed by drilling out, which is a costly and time consuming procedure.
A retrievable bridge plug has slip elements and packer elements. The slip elements are used to grip the inside surface of the well casing, thereby preventing the bridge plug from moving up or down the casing. The packer elements engage the inside surface of the well casing to provide the requisite seal. The bridge plug is set in position by radially extending the slip elements and the packer elements. To retrieve the bridge plug from the well casing, the slip elements and packer elements are retracted and disengaged from the casing inside surface.
During workover operations, a pressure differential above and below the bridge plug may be developed. It is desirable to equalize this pressure differential before the slip and packer elements are disengaged. Equalization prevents the loss of control over the bridge plug, wherein the tool may be blown up or down a well casing in response to the pressure differential, after the slip and packer elements have been released.
In prior art bridge plugs, the tool is released by manipulations that are also used to equalize pressure. For example, in Mayland, U.S. Pat. No. 4,432,418, pressure is equalized by pulling up on the tool. Continued upward force causes the slip and packer elements to release. The bridge plug thus can be released while still under a pressure differential. What is needed is a bridge plug that can be manipulated to equalize pressure without the possibility of releasing the slip and packer elements.